Hello @Nexgolit and welcome to the forum!
Duplicati does not directly provide a package for Arch/Manajaro from what I can see (I don’t run either).
What user is Duplicati using (e.g. look by ps -ef | grep Duplicati? If duplicati that’s the cause.
https://aur.archlinux.org/cgit/aur.git/log/duplicati.service?h=duplicati2-beta
2017-10-22 duplicati.service should run as duplicati user now.
shows where that change happened, in at least one potential source. I don’t know where you got yours. Perhaps the maintainer of your package could explain why things are that way, e.g. was it for security?
Possibly putting your user (only one?) in group duplicati would let Duplicati get to files by group access, assuming your files tend to give read access to group (I don’t know what sort of umask you’re running).
Having Duplicati run as root would solve R/W access and is good if you backup multiple users at once.
Installing Duplicati on Linux (Arch / Manjaro) has a solution that possibly runs as root, but I can’t test it…